My impression of form factor in the sense of "size, shape, and design of an electronic product" is that it is what Wilson Follett, Modern American Usage (1966) calls a "popularized technicality":
popularized technicalities. ... In our time what is technical and professional is in high repute; what comes from the amateur is regarded as amateurish. Consequently many phrases have been borrowed from the sciences, the techniques, and the professions to adorn and lend expressiveness o ordinary prose. The choice and application of these words and phrases have naturally not been controlled by the experts; the transfer has been indeed amateurish, and examination shows that a good many of the new terms simply duplicate or replace simple words long in use.
Oxford Dictionary of Computing, sixth edition (2008) gives two definitions—one simple and one technical (and difficult)—of the term as used in computer science:
form factor 1. The shape of a piece of equipment expressed either in height, width, and depth, or in terms of a standard item such as a 3½-inch disk drive or a 19-inch rack. 2. The fraction of radiation diffusely emitted from one surface that is received by another. Form factors are used in radiosity calculations and are strictly geometric quantities whose values depend only on the slope and relative location of the surfaces in the scene. Form factors are independent of view and hence do not have to be recomputed for a change of view.
Today, form factor is used extensively in the computer/electronics industry to refer to something like "shape or design [of a product]." At the computer magazines where I worked for many years, some journalists used it constantly in things like cell phone reviews, where (for example) a particular phone would be described as having models were described as having a "clamshell" or a "candybar" or a "slab" form factor. Wikipedia has an entire article on "Form factor (mobile phones)," which begins with this definition:
The form factor of a mobile phone is its size, shape, and style, as well as the layout and position of its major components.
One of the earliest Google Books matches for form factor to use the term as a synonym for size is Jagdish Mehra, The Physicist's Conception of Nature (1973), which nonetheless applies the term o "charged bodies" not to product models:
The result of this more detailed analysis agrees with that of Bloch and Nordsieck except for the observation that the probability of very small energy losses depends on the size (form-factor) of the charged body, in such a manner as to 'render immediate application of real electrons impossible'.
An early instance of equating form factor with shape occurs in C.J. Abend, "Product Appearance as Communication," in Sensory Evaluation of Appearance of Materials: A Symposium (1973):
Another form factor [of a hand ax] is the pointed and wedge shape of the head which is directional and arrow-like in nature; this is a further visual clue with respect to direction of work. The back of the head is blunt, which reveals thickness and heft; massiveness, and potential inertia are made visible.
But the earliest instance that I could find of form factor as a popularized technicality used in connection with a product's size, shape, and design is from an advertisement for four Collins aviation instruments in Flying Magazine (October 1971):
IMPROVED RELIABILITY—At least twice the predicted mean time between failures.
TOP PERFORMANCE—Collins traditional, uncompromised standards of quality.
IDEAL FORM FACTOR—Minimizes rack space; fits previously unusable space in aircraft.
LATEST CAPABILITIES—Full channeling for proposed future requirements.
Here form factor may retain the cachet of its more technical senses in physics, electronics, and elsewhere, but it has lost their complicated meanings. It has become, in short, a ten-cent Madison Avenue term for "size, shape, and/or design."